I think these Dongles will offer higher quality than an 500$ Desktop Device.
Desktop Devices are very complex to develop and manifacture. One of the main reasons is that, unlike dongles, the have a large Power Supply with AC/DC Conversion from 100-240V and so on.
If you look at the FiiO K9 Pro Board
You can see that almos half of the board is just used, to clean the power source. And because it is complex, it is expensive. To only way to save money/make them cheap is to not bulid them complex, but then they will suffer from the unclean power supply. And not just the power supply, this is just one of many aspects. Large Desktop Amps have lots of components that induct noise into the circuit like Bluetooth Receivers, ADCs for COAX and optical and so on.
Its not one single thing, its a sum of things that will result in an very high complexity that will result in an high noise floor.
At a certain point, ther is no way around separating DAC and Amp.
For example if you use a separate DAC+Amp, you can have 6W @ 32 Ohm (3-times of the K9 Pro ESS) with less than <2µV of Noise.
The PCB of an very well implemented DAC is already bigger than the FiiO K9 Pro which is DAC+Amp. The PCB of an good implemented Amp is roughly the same size as the DAC.
This is the S.M.S.L. D400 EX
Its almost the double size of the FiiO K9 Pro ESS and its just a DAC alone.
The more Power you want/need, the more complex is it to handle that power. As soon as wall sockets come into play, you'll spend half of the PCB just cleaning the power from the wall socket. That is why a lot of DAC/AMP like the DMP-Z1 (and others) use an battery array to store the electricity. But thats even more expensive and causes its own set of problems.
So bottom line, if you really really want an desktop setup, invest once and wisely to save yourself from constant upgrades and issues. I am a big fan of buy once and be happy.